Make CI fail if any unused mix dependency

Author: grzuy

.github/workflows/elixir.yml

@@ -22,6 +22,8 @@ jobs:
         restore-keys: ${{ runner.os }}-mix-
     - name: Install mix dependencies
       run: mix deps.get
+    - name: Check unused mix dependencies
+      run: mix deps.unlock --check-unused
     - name: Check code format
       run: mix format --check-formatted
     - name: Install yarn dependencies